Abstract: We use neutron scattering to study the Pr$^{3+}$ crystalline electric fieldCEF excitations in the filled skutterudite PrOs$ 4$As$ {12}$. By comparingthe observed levels and their strengths under neutron excitation with thetheoretical spectrum and neutron excitation intensities, we identify thePr$^{3+}$ CEF levels, and show that the ground state is a magnetic$\Gamma 4^{2}$ triplet, and the excited states $\Gamma 1$, $\Gamma 4^{1}$and $\Gamma {23}$ are at 0.4, 13 and 23 meV, respectively. A comparison of theobserved CEF levels in PrOs$ 4$As$ {12}$ with the heavy fermion superconductorPrOs$ 4$Sb$ {12}$ reveals the microscopic origin of the differences in theground states of these two filled skutterudites.